1. Field of the Invention
The invention relates to a RF induction plasma source generating apparatus, and more specifically to a RF induction plasma source generating apparatus which generates a stabilized plasma and sustains the stabilized plasma by maintaining the plasma pressure from several hundreds Torr to several thousands Torr with attachment a buffer nozzle cap, upper metallic blocking films and lower metallic blocking films to a nozzle cap, plasma tube and RF induction coils, respectively for separating the plasma source generating apparatus from the chamber.
2. Description of the Prior Art
Generally, group III-V nitride compound semiconductors have been studied over recent 8 years as they are superior to other than semiconductor materials. Of the epitaxial growth technologies for epitaxially growing the group III-V nitride, researches have been developed which realize the benefits of ultra high vacuum molecular beam epitaxy capable of epitaxially growing with accuracy amounting to several atomic layer by heterogeneous junction of multi-epitaxial layer.
However, N.sub.2 gas does not react with other materials, a plasma source is necessary to decompose this gas and introduce an atomic nitrogen. The conventional plasma source generating apparatus is similar to FIG. 1, but without grid electrode. The apparatus has been often used in plasma assisted molecular beam epitaxy (PAMBE) growth of GaN type compound semiconductors, and the content of which is described in the preceding patents and articles.
The preceding patents are as follows:
U.S. Pat. No. 5,637,146 published on Jun. 10, 1997 and entitled "Method for the growth of nitride based semiconductors and its apparatus"; U.S. Pat. No. 4,268,711 published on May 19, 1991 and entitled "Method and apparatus for forming films from vapors using a contained plasma source"; and U.S. Pat. No. 5,651,825 published on Jan. 29, 1997 and entitled "Plasma generating apparatus and plasma processing apparatus".
The preceding article is the article entitled "The effect of the III/V ratio and substrate temperature on the morphology and properties on GaN- and AIN-layers grown by molecular beam epitaxy on Si(111)" by M. A. Sanchez-Garcia, et. al., J. Crystal Growth (183, 23), 1998.
However, PAMBE growth of GaN is not so successful in view of the application of device. As a this cause, various situations may be considered but principally resulting from the defects in the surface of the substrate by the kinetic energy of the ion created by plasma potential.
The energy of ion has the distribution with Full-Width at Half maximum (FWHM) being corresponding to about 5 eV, and can has the distribution having the average energy amounting to 2-3 eV at minimum to 30 eV at maximum depending on the flow rate of the used N.sub.2 gas and the supplied RF power. In the conditions of RF plasma mainly used in the epitaxial growth, since the ions which arrive at the substrate spaced 15 cm apart from them amount to about 10.sup.12 cm.sup.-2 s.sup.-1, even when the possibility of the defect creation is 0.1% in case of the epitaxial growth in the growth rate of 100 nm/hr, the defect corresponding to the density of 10.sup.17 cm.sup.-3 is created. This density defect greatly degrades the quality of the epitaxial layer.
In view of avoiding the above-mentioned problem, various methods have been proposed.
One of the methods is a method wherein the ions are removed the outside by application of the electric field. This method is described in the article entitled "The energetics of the GaN MBE reaction: a case of meta-stable growth", by N. Newman, J. Cryatal Growth (178, 102, 1997).
FIG. 1 is a cross-sectional view of the prior art plasma source generating apparatus adopting this method.
As shown in FIG. 1, the prior art plasma source generating apparatus is comprised of plasma tube 1, nozzle 2, RF induction coils 3, a grid electrode 4 placed on the top of the nozzle 2, and a power source 5 for supplying a voltage Vg to the grid electrode.
Now, the operations of the thus constructed plasma source generating apparatus will be briefly explained. When the grid electrode 4 placed on the top of the nozzle 2 is supplied with the voltage Vg by the power source 5, the ions of plasma source being sprayed through the nozzle 2 are removed in dot arrow shown in FIG. 1 via the grid electrode 4.
Such a prior art plasma source generating apparatus has a merit that the presence of ions can be controlled by the voltage applied to the grid electrode 4 and charge particles such as electrons can be removed.
However, the prior art plasma source generating apparatus requires a additional power source for supplying the voltage of 300 V or more to substantially removing the ions arriving at the substrate and causes the sputtering phenomenons with the ions accelerated by the voltage being impinged on the chamber around the grid electrode or the cooling device, thereby the impurities being transferred to the substrate, eventually degrading the quality of the epitaxial layer. And, the prior art plasma source generating apparatus has disadvantage in that the gases and particles passing through the nozzle of the predetermined size or more have the shape being spreaded out outside of the nozzle by the wide angle of the motion direction, thereby reducing the density of the ions arriving at the substrate.
FIG. 2 is a cross-sectional view of the prior art plasma source generating apparatus provided with fine nozzles having the size of 0.5 mm and less.
As shown in FIG. 2, the prior art plasma source generating apparatus is comprised of a plasma tube 10, fine nozzles 11, and RF induction coils 12.
The thus constructed plasma source generating apparatus utilizes the principle wherein the apertures of fine nozzles 11 serve as a high plasma potential barrier with this apparatus being provided with fine nozzles 11 of several hundreds, in order for the ions not escaping outside of nozzles.
This principle is described in the article entitled "Characteristics of a mesh-bias-controlled electron cyclotron resonance plasma for the growth of gallium nitride of epitaxial films", by K. Yasui, et al, J. Vac. Sci. Technol. (A16, 369, 1998).
However, such a prior art plasma source generating apparatus has a problem in that the plasma potential barrier is still not perfect. And, it is considered that the prior art plasma source generating apparatus has a high cost and the low efficiency in preventing ions from escaping in case of producing the nozzle of 0.5 mm size according to a laser process. Therefore, if the diameter of fine nozzle in PBN (Pyrolitic Boron Nitride) tube is less than 0.5 mm, the efficiency of preventing ions from escaping slightly becomes high, but there are limitations in producing fine nozzles of several thousands. Also, since the electric field may be transferred outside near nozzle of the PBN tube when RF power is high, the object of removing the ion cannot be accomplished